Abnormalities of bronchopulmonary resistance mechanisms provide the basis by which most bacterial infections become established and progress in the lung. Bronchopulmonary defense mechanisms focus principally on the phagocytic capabilities of the alveolar macrophage system, secondarily supported by alveolar and bronchial transport systems. This project is a study of basic cellular mechanisms which focus on the alveolar macrophage, and its immunological supporting apparatus. Specific projects are directed toward determining the immunologic apparatus of the lung and particularly the immune mechanisms that enhance pulmonary alveolar macrophage activity. The project is also directed to an understanding of the cellular basis of virus-bacterial interactions of the lung by studying how the immunologic apparatus prevents the virus induced defect. Bibliographic references: Jakab, G.J., and Green, G.M.: Variations in Pulmonary Antibacterial Defenses among Different Species of Experimental Animals. Infect. Immun. 11:601, 1975. Harrow, E.M., Jakab, G.J., Brody, A.R., and Green, G.M.: The Pulmonary Response to a Bacteremic Challenge. Amer. Rev. Resp. Dis. 112:7, 1975.